1. Field of the Invention
The present invention relates to detecting the location of a target relative to the position of a tracking device, and more specifically, it relates to a small, non-contact laser sensor for detecting target position.
2. Description of Related Art
The ability to track the unpredictable path of objects or locate the position of objects within a three-dimensional space has many applications. This can range from merely identifying arrival at a particular location, to obtaining additional information about the target, to finding the position of a stationary object at an unknown position, to actively following the position of a moving object as it moves. For example, tracking people would enable their motions to be simulated and analyzed in software, which could be used for a wide variety of applications, including providing valuable information for physical therapy or for virtual reality games. Located on cars, a tracking sensor could provide the location of the car relative to the road, warning the driver of danger. Conversely, the tracker could be on the road, illuminating targets on cars which return information indicating presence or on the type, license, etc., of the car. Other examples include tracking the path of an instrument as it is inserted into a hazardous area or following an automated manufacturing tool as it manipulates a part. In a highly automated industrial environment, tools are often left to perform their functions with little or no feedback as to their performance. Costly mistakes can result when a tool or item is placed in the wrong location. One way of avoiding the mis-location of objects is to track their locations with a sensor.
There are many different types of sensors (laser, magnetic, ultrasound, etc.) which can provide information on the location of an object. Most of those sensors provide information on only one dimension, usually range. While range information can be very useful, it is inadequate to monitor objects with multi-dimensional freedom of motion. To track several axes of motion, several range sensors can be combined to provide more degrees of information, but generally this is impractical, adding cost and difficulty in maintaining orthogonality between the sensors. Some sensors provide more than one degree of information. Several types provide two dimensional information, e.g. a camera which locates lateral position of an object in some limited field of view. Only a few provide information in three dimensions (lateral translation and range). These three dimensional (3-D) sensors generally use a laser to scan a field-of-view, collecting diffuse light to compute a 3-D image of objects. These types of sensors are good for identifying the type and orientation of objects, but they are hardware and software intensive, making them slow and expensive for target tracking. Because they use diffuse reflectors, their range is limited to tracking objects in small volumes.
A laser coordinate measurement system is a fast and accurate 3-D tracker. It tracks a special retroreflector target (cooperative target) with one to several lasers, obtaining accuracies of a few micrometers in a large volume (many cubic meters), but these devices can be very expensive ( greater than $100,000) and large ( greater than 1 cubic foot). Furthermore, a coordinate measurement system must record a starting reference point to achieve high accuracy, which may be impractical in many applications. Another draw back of this type sensor is its inability to track more than one target at a time.
Accordingly it is an object of the invention to provide a small, inexpensive, non-contact laser sensor which tracks one or more targets.
It is also an object of the present invention to provide a sensor to track a retroreflective target in three dimensions.
It is another object of the invention to measure up to six degrees of target position.
The present invention is a small, inexpensive, non-contact laser sensor which tracks retroreflectors fixed to a target. The tracker includes a laser which produces an output beam. The beam passes through a linear polarizer tilted at an angle so that the beam has a linear polarization. The beam then passes through a quarater wave plate to produce a beam with circular polarization. The beam passes through an optical system to the target and a reflected beam is directed back through the quarter wave plate to produce a linear polarization orthogonal to the initial polarization so that the beam cannot pass back through the polarizer but is instead reflected to a detector.
In one embodiment, the tracker""s laser beam is formed into a plane of light which is swept across the space of interest. When the beam illuminates a retroreflector on the target, some of the light returns to the tracker. The intensity, angle, and time of the return beam is measured to calculate the three dimensional location of the target. With three retroreflectors on the target, three points on the target are measured, enabling the calculation of all six degrees of target position. Furthermore, this sensor is capable of tracking more than one target at a time.
In an alternate embodiment, the tracker produces a diverging laser beam which is directed towards a fixed position and senses when a retroreflective target enters its field of view. The target can be formed of a ball lens with a bar code on one end so that as the target moves through the field the ball lens causes the laser beam to scan across the bar code.
Until now, devices for three-dimensional tracking of objects in a large volume have been heavy, large, and very expensive. Because of the simplicity and unique characteristics of this tracker, it is capable of three-dimensional tracking of one to several objects in a large volume yet it is compact, light-weight, and relatively inexpensive.